The monitoring of plasma soluble interleukin-2 receptor (S IL-2R) concentrations has been proposed in organ transplantation, especially to detect early manifestations of rejection. In organ transplantation, immune activation occurs in various circumstances such as rejections and infections. We performed S IL-2R determination 3 times a week in the sera of 106 patients undergoing kidney and/or pancreas transplantation. In kidney transplantation, S IL-2R was increased before the transplant. It also increased under prophylactic and especially under curative anti-rejection OKT3 or ATG therapy. In 90% cases, S IL-2R increased 2 to 4 days before creatininemia rise. In the other 10% cases, no correlation could be found with any clinical status modification. S IL-2R concentrations never increased in isolated acute tubular necrosis or in cyclosporine A (CsA) nephrotoxicity. In pancreas transplantation, the correlation between S IL-2R concentrations and possible pancreas rejection, was very poor. During cytomegalovirus (CMV) infection, only 50% patients with clinical CMV manifestations had high concentrations of S IL-2R. During Dihydroxy Propoxy Methyl Guanine (DPHG = Ganciclovir) treatment, S IL-2R still increased at the beginning, then it decreased progressively when therapy was efficient on CMV infection. The monitoring of S IL-2R concentrations may be useful in the weeks following organ transplantation provided that results are interpreted in the context of clinical and other laboratory findings, particularly with the renal function status and creatininemia.